1. Field of the Invention
The present invention relates to an optical communication. More particularly, the present invention relates to a bi-directional path switched ring (referred to as “BPSR” hereinafter) optical transmission node.
2. Description of the Related Art
When forming a network by means of a BPSR optical transmission node which has a BPSR configuration and uses one optical fiber, the configuration of the network is simple, a switching time required for a projection is short, and performance of the optical fiber is excellent.
A conventional 2×2 interleaver 10 is described as follows with reference to FIG. 1. The conventional 2×2 interleaver is an optical filtering element which directs a received signal to diagonal and straight ports when the received signal has odd and even channels, respectively.
As shown in FIG. 1, when an odd channel signal enters the 2×2 interleaver 10 through a port A 11, the interleaver 10 directs the odd channel signal to a port D 14. When an even channel signal enters the 2×2 interleaver 10 through the port A 11, the interleaver 10 directs the even channel signal to a port C 13. Consequently, when both of the odd channel signal and the even channel signal enter one port, namely, the port A of the 2×2 interleaver 10, the odd channel signal and the even channel signal are directed to the port D 14 and the port C 13 thereof, respectively. Accordingly, the signals are separated by channels. When the odd channel signal and the even channel signal are inputted to the port A 11 and the port B 12, respectively, both of the odd channel and the even channel signal are directed to the output port D 14. Inputs to ports B, C, and D operate in the same manner as described above with regard to the input to the port A. The 2×2 interleaver 10 directs in a common direction for subsequent amplification odd channel signals and even channel signals traveling in different directions and then re-directs the signals in their original direction after amplification. Consequently, the 2×2 interleaver 10 is useful for optical transmission using a bi-directional line.
FIG. 2 shows the configuration of a conventional BPSR optical transmission node using 2×2 interleaver.
The conventional BPSR optical transmission node using 2×2 interleaver is a ring node which directs an odd channel and an even channel to diagonal and straight directions through one optical fiber, respectively, and advantageously has a simple structure.
The operation of the conventional BPSR optical transmission node will be described with reference to FIG. 2. A first optical line 21 receives an odd channel signal and directs the odd channel signal to an input terminal of an amplifier 24 via an interleaver 23. A second optical line 22 receives an even channel signal and directs the even channel signal to the input terminal of the amplifier 24 via the interleaver 23. The amplifier 24 amplifies the odd channel signal and the even channel signal from the first and second optical lines 21, 22, respectively, and provides the amplified odd and even channel signals to an optical add/drop multiplexer (referred to as a “ADM” hereinafter) 25. The ADM 25 is a main element of a wavelength division multiplexing transmission system and is coupled at a predetermined point between a transmitting terminal and a receiving terminal. The ADM 25 selectively adds or drops each channel of the amplified signals in succession from the amplifier 24. The odd channel signal from the ADM 25 is directed to the second optical line 22 through the interleaver 23. The even channel signal from the ADM 25 is directed to the first optical line 21 through the interleaver 23.
According to the BPSR optical transmission node using 2×2 interleaver, a single amplifier amplifies optical signals which are directed to both directions. Demand for BPSR optical transmission nodes has risen due to their simple configuration. However, when something is wrong at least with either of the first or second optical lines 21 and 22, it is difficult to detect the wrong state, since the odd channel signal and the even channel signal are simultaneously detected during an amplifying cycle through the first optical line 21, the second optical line 22, and the interleaver 23.
Furthermore, it is difficult to detect the occurrence of an obstacle such as a cutting of the optical fiber. Since a reflected wave passes through an amplifier, the wrong transmission of a signal occurs due to the reflected wave. In general, the reflected wave is not influenced significantly by a Rayleigh back scattering. However, the reflected wave is a reflection of a signal that has been amplified, and, as such, has sufficient intensity to interfere with signals not yet amplified and therefore relatively weak.
The amount of reflection generated from an optical fiber normally varies according to the kind of the optical fiber and the wavelength of the signal in use. The reflected wave has an amplitude that has been reduced by about 32 dB due to Rayleigh back scattering. An ideal interleaver perfectly removes the reflected wave generated from the optical fiber by the Rayleigh back scattering. However, a real interleaver reduces the reflected wave by about 20 dB. Consequently, the intensity of the reflected wave which is inputted to the 2×2 interleaver has been reduced by about −52 dB.
For example, the span of a standard single mode optical fiber is 40˜50 km. Assuming a span loss of the optical fiber in the BPSR optical transmission node of about 10˜15 dB, a destination signal, i.e., desired signal, that has undergone a span loss of about 10˜15 dB arrives at the amplifier input terminal. Accordingly, the reflected wave reduced by about −52 dB has an intensity of about 37˜42 dB below that of the destination signal. Since a signal having an intensity of at least 30 dB less than that of a desired signal does not much affect the transmission characteristic of the desired signal, a reflected wave having an intensity of 37˜42 dB below that of the destination signal has a little influence on the destination signal.
However, when the obstacle is due to cutting of the optical fiber, the intensity of the reflected wave is attenuated merely by about 14 dB, so that after passing through the interleaver, the intensity of the undesirable signal has decreased by a total of 34 dB. Assuming a span loss in the optical fiber of 10˜15 dB, the intensity of the reflected wave at an input terminal of the amplifier becomes −24˜−19 dB in comparison with that of the destination signal. Thus, the wave reflected due to the cutting of the optical fiber detrimentally affects destination signals, i.e. lowers transmission quality.
A conventional BPSR optical transmission node using four interleavers as shown in FIG. 3 is utilized to detect obstacles, such as the cutting of an optical fiber, and to mitigate the influence of the reflected wave during the occurrence of an obstacle.
Referring now to FIG. 3, the conventional BPSR optical transmission node includes four 1×2 interleavers each having a number of inputs or outputs one less than that of the 2×2 interleaver.
The operation of the 1×2 interleaver will be described by using the 2×2 interleaver shown in FIG. 1. In a first 1×2 interleaver 33, an odd channel signal is inputted through a port A, and the inputted odd channel signal is directed to a diagonal port D. An even channel signal is inputted through a port C, and the inputted even channel signal is directed to a straight port A. However, a port B is not used in first 1×2 interleaver 33. In a second 1×2 interleaver 34, an odd channel signal is inputted through a port A, and the inputted odd channel signal is directed to a diagonal port D. An even channel signal is inputted through a port D, and the inputted even channel signal is directed to a straight port B. However, a port C is not used in the second 1×2 interleaver 34.
In a third 1×2 interleaver 35 operating as a multiplexer, an odd channel signal is inputted through a port A, and the inputted odd channel signal is directed to a diagonal port D. An even channel signal is inputted through a port B, and the inputted even channel signal is directed to a straight port D. However, a port C is not used in the third 1×2 interleaver 35. In a fourth 1×2 interleaver 36 operating as a demultiplexer, an odd channel signal is inputted through a port A and the input odd channel signal is directed to a diagonal port D. An even channel signal is inputted through a port A, and the inputted even channel signal is directed to a straight port C. However, a port B is not used in the fourth 1×2 interleaver 36.
In the operation of the conventional BPSR optical transmission node using four 1×2 interleavers 33, 34, 35, and 36, a first optical line 31 receives an odd channel signal. The received odd channel signal is provided to an amplifier 37 via first and third interleavers 33, 35. A second optical line 32 receives an even channel signal. The received even channel signal is provided to the amplifier 37 via second and third interleavers 34 and 35. The amplifier 37 simultaneously amplifies the odd channel signal and the even channel signal from the first and second optical lines 31 and 32, respectively. The amplified odd and even channel signals are passed through an ADM 38. The amplified odd channel signal from the ADM 38 is directed to the second optical line 32 via the fourth interleaver 36 and the second interleaver 34. The amplified even channel signal from the ADM 38 is directed to the first optical line 31 via the fourth interleaver 36 and the first interleaver 33.
When an obstacle occurs during the above operation, the reflected wave passes through two interleavers, namely, either the first and third interleavers 33, 35 or the second and third interleavers 34, 35 before amplifying the reflected wave. Accordingly, the intensity of the reflected wave is reduced by about 40 dB by the two interleavers so that it is reduced by an additional 20 dB in comparison with the interleaver shown in FIG. 2. Consequently, even though the intensity of the reflected wave is decreased a mere 14 dB due to the cutting of an optical fiber, and assuming a span loss in the optical fiber of 10-15 dB, the intensity of an undesirable signal in an input terminal of the amplifier falls 39˜44 dB below that of the destination signal. The reflection therefore does not affect significantly the transmission characteristic of a desired signal. Also, since only the odd channel signal is directed to the first interleaver 33 and the third interleaver 35, and the fourth interleaver 36 and the second interleaver 34, photo detectors are installed between the first interleaver 33 and the third interleaver 35, and between the fourth interleaver 36 and the second interleaver 34 and detect whether or not obstacles occur during a transmission of the odd channel signal. Similarly, since only the even channel signal is directed to the second interleaver 34 and the third interleaver 35, and the fourth interleaver 36 and the first interleaver 33, photo detectors are installed between the second interleaver 34 and the third interleaver 35, and between the fourth interleaver 36 and the first interleaver 33 and detect whether or not obstacles occur during a transmission of the even channel signal.
Although the conventional BPSR optical transmission node shown in FIG. 3 detects the occurrence of an obstacle and mitigates the influence of the reflected wave during the occurrence of an obstacle, four interleavers are used, causing the construction thereof to be complicated. Furthermore, a temperature control circuit normally used in order to stably operate the four interleavers adds complexity to and increases the size of the conventional BPSR optical transmission.